1. Field of the Invention
The invention is directed to a tensioning device for belts and the like, in particular on motor vehicle engines, including a belt diversion wheel, which is rotatably supported on one end of a swivel arm, the other end of the swivel arm being pivotably supported between two end locations about a bearing bush connected to a rotationally fixed housing part. A swivel arm housing is connected to the swivel arm and at least partly surrounds the bearing bush concentrically, leaving a space in between. A friction bushing comprising an elastically deformable material is disposed in the intervening space, while a helical spring is disposed in the annular gap that still remains, the ends of the spring being connected to the bearing bush or to the swivel arm housing in a fixed so as to prvent relative rotation with respect to one another. At least at some points, the helical spring is in contact with the friction bushing.
2. The Prior Art
A device of this kind is known from U.S. Pat. No. 4,473,362. In this previously known device, the helical spring is disposed reltive to the friction bushing in such a way that vibrations of low amplitude are damped by a damping bushing of elastomeric material, while motions of high amplitude are damped via the helical spring and the frictional contact between the friction bushing and the housing moving contrary to it, in such a way that the damping force decreases whenever the swivel arm is moved away from a first end position toward the second end position.
This constructional embodiment would appear, in purely static terms, to be entirely satisfactory in meeting the basic demands made of such a device. However, a more detailed consideration of the dynamic properties, both theoretically and in experimental tests, shows that such a previously known system exhibits an uncushioned, undamped and correspondingly rigid behavior, especially in the presence of low-amplitude vibration. This occurs because before a cushioning or damping effect can occur, the static friction between the friction bushing and the housing must be overcome (this static friction is of course substantially greater than the sliding friction that arises after it has been overcome), and because adequate damping cannot be attained by the elastic deformation of the damping bushing alone.
In the previously known constructions, a virtually constant tensioning force is accordingly sought, while the actual requirements in practical operation are ignored; in that case it is unavoidable that the belt must itself absorb a majority of the shocks and recoiling, which leads to internal friction and relatively major heating of its elastomer components, causing degradation of the elastomer components and hence lessening their durability. In this previously known system, the spring is also incapable of fast enough follow-up, so that when minor shocks occur, the inertia of the system causes belt slippage, which in principle also causes heating and thus shortens the life of the belt.